NWA 2898

H7 or Meta-H

Found 2003
no coordinates recorded A single meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and Click on Term to Read More weighing 136 g was found in the desert region of Northwest Africa, and subsequently purchased by S. Ralew. The stone was analyzed and classified at Humboldt University, Berlin (A. Greshake), and was determined to be a completely recrystallized H chondriteOrdinary chondrites with a high content of free Ni-Fe metal (15-19 vol. %) and attracted easily to a magnet. Their main minerals are olivine (Fa16-20) and the orthopyroxene bronzite (Fs14.5-18.5), earning them their older name of bronzite chondrites. Chondrules average ~0.3 mm in diameter. Comparison of the reflectance spectra of Click on Term to Read More lacking any relict chondrules—an H7 chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More. The olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion Click on Term to Read More Fa (17.7) and pyroxeneA class of silicate (SiO3) minerals that form a solid solution between iron and magnesium and can contain up to 50% calcium. Pyroxenes are important rock forming minerals and critical to understanding igneous processes. For more detailed information, please read the Pyroxene Group article found in the Meteoritics & Classification category. Click on Term to Read More Fs (15.6) values are consistent with the H chondrite group. On an oxygen 3-isotope diagram, the plot of NWA 2898 is also consistent with an origin on the H-chondrite parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. Click on Term to Read More (I. Franchi and R. C. Greenwood, OU).

Northwest Africa 2898 is one of a very few H7 chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More to be classified, five of which were found in Antarctica, a few others being found in Northwest Africa (NWA 2835, 4226, and 4229). While these meteorites presently constitute the currently accepted H7 petrologic grouping, it should be noted that certain highly metamorphosed meteorites with very close chemical and isotopic similarities to the H-group chondrites have been assigned to the recently proposed metachondriteTerm used to describe a metamorphosed chondrite. Also referred to as a type 7 chondrite. Metachondrites are texturally evolved rocks derived from chondritic precursors and some have been classified as primitive achondrites. Click on Term to Read More classification (Irving et al., 2005); e.g., NWA 2353, NWA 2635, and 3145, all three of which are thought to be paired with the H7 NWA 2835.

The thermal history of the H chondrite parent body was calculated by Harrison and Grimm (2010) based on cooling rate data and closure times. They found that the object accreted over a short time period of 2.2 m.y. In an alternative viewpoint, Monnereau et al. (2012) determined a more rapid accretionAccumulation of smaller objects into progressively larger bodies in the solar nebula leading to the eventual formation of asteroids, planetesimals and planets. The earliest accretion of the smallest particles was due to Van der Waals and electromagnetic forces. Further accretion continued by relatively low-velocity collisions of smaller bodies in the Click on Term to Read More time period of 0.1–0.2 m.y. while 26Al was still extant. Moreover, Sokol et al. (2007) concluded that accretion of the ~150 km H-chondrite parent body occurred relatively late after most radiogenic 26Al had decayed, at least 2 m.y. after CAISub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they Click on Term to Read More formation; it was probably heated by continuing impacts. It is generally considered likely that the asteroid eventually formed an insulated ‘onion-shell’ structure with a diameter of 150–260 km. The H-chondrite parent body was composed approximately (by volume) of 84%, 10%, and 6% of type 6, type 4/5, and type 3 material, respectively.

Amelin et al. (2005) utilized thermal models to calculate the progressive increase in petrologic types from the coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel. Click on Term to Read More to the surface as follows: from the core outward to a distance of 44.9 km is type 6 material: between 44.9 km and 48.9 km is type 5 material; between 48.9 km and 56.9 km is type 4 material; and from 56.9 km to the surface at 92.5 km is type 3 material. Peak temperatures were determined to be 865–1000°C, 675–865°C, and <675°C for type 6, type 4/5, and type 3 material, respectively. The higher petrologic types were excavated at depth by impact, forming craters measuring tens of km-wide and reaching depths of 5.6–11.2 km on their 200 km diameter model. FissionBreaking apart of a body into smaller fragments. In nuclear physics, fission refers to splitting of a heavy atomic nucleus into two or more lighter nuclei with an associated release of energy. The mass of the nucleus before fission is greater than the combined masses of the resulting fragments; the Click on Term to Read More track thermochronometry indicates that type 7 chondrites cooled more slowly at greater depths than did those with lower petrologic types (Trieloff et al., 2003). Consequently, type 7 chondrites experienced a longer period of thermal metamorphism within this interior layer, and now they exhibit extensively recrystallized textures that are transitional to an achondriteAn achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent body reached a sufficient size that through heating due to radioactive decay of 26Al (aluminum isotope) and gravitational Click on Term to Read More classification.

Importantly, a complex cooling history for the higher petrologic typeMeasure of the degree of aqueous alteration (Types 1 and 2) and thermal metamorphism (Types 3-6) experienced by a chondritic meteorite. Type 3 chondrites are further subdivided into 3.0 through 3.9 subtypes. H chondrites (5/6) was suggested from thermometric studies conducted by Ganguly et al. (2012). They reconciled data from calculations of two-pyroxene thermometers with the Ar–Ar, Pb–Pb, and Hf–W closure temperatures of select minerals to determine a cooling history consistent with very rapid cooling between ~800°C and 450°C, followed by a very slow cooling stage, and then another rapid cooling stage. By contrast, those H chondrites with lower petrologic types experienced a steady state of very rapid cooling. It was proposed that this scenario was more consistent with a collisional disruption and re-accretion of the parent body as opposed to a smoothly transitional ‘onion shell’ model.

Type 7 ordinary chondrites were originally defined by Dodd et al. (1975) according to specific petrographic characteristics. They listed three metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More criteria to distinguish between petrographic types 6 and 7:

1. the presence of poorly defined chondrulesRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at  ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine Click on Term to Read More in type 6, but only relict chondrules in type 7
2. low-Ca pyroxenes contain no more than 1.0 wt% CaO (1.0 wt% = ~1.9 mol% Wo) in type 6, but more than 1.0 wt% in type 7; conversely, the CaO content of high-Ca pyroxenes decreases from type 6 to type 7
3. feldsparAn alumino-silicate mineral containing a solid solution of calcium, sodium and potassium. Over half the Earth’s crust is composed of feldspars and due to their abundance, feldspars are used in the classification of igneous rocks. A more complete explanation can be found on the feldspar group page. Click on Term to Read More grains gradually coarsen to reach a size of at least 0.1 mm in type 7

In the intervening years since Dodd et al. proposed their classification parameters, additional type 7 chondrites have been found and studied. As a result of more recent studies, it was proposed by Wittke and Bunch (pers. comm., 2004) that a type 7 category should not comprise meteorites containing any relict chondrules, but rather, should represent a metamorphic extreme in which no sign of chondrules remains. This would lump those meteorites containing ‘poorly defined’ chondrules and ‘relict’ chondrules into the type 6 category.

In further contrast to Dodd et al., Wittke and Bunch (2004) suggest that the relative size of all of the silicates, instead of only the feldspar grains, would provide a better gauge of a petrographic type 7 since silicates attain an equigranular texture only under the highest metamorphism. They have also discovered that simple twinning of plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More occurs only in type 7, and suggest that this could be utilized as an additional parameter. Beyond that, it was revealed that modal metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More contents decrease significantly during late metamorphic stages; i.e., low-Ni metal, as well as pyroxenes, are consumed to produce olivine, resulting in only small amounts of Ni-rich metal along with lower amounts of orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur Click on Term to Read More and clinopyroxene compared to those amounts present in lower metamorphic grades.

H7 chondrites have an uneven distribution of metal and silicates, and a heterogeneous grain size distribution. The coarse silicates might be remnants of the original chondrules. Research has been published which identifies specific characteristics that distinguish type-7 chondrites from metachondrites (primitive achondrites). The following characteristics are typically observed in metachondrites (Ford et al., 2004):

1. an equigranular (igneous) texture with no extensive segregation
2. experienced temperatures to levels necessary for FeNi-metal, FeS, and silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the partial meltingAn igneous process whereby rocks melt and the resulting magma is comprised of the remaining partially melted rock (sometimes called restite) and a liquid whose composition differs from the original rock. Partial melting occurs because nearly all rocks are made up of different minerals, each of which has a different melting Click on Term to Read More (~1200°C, perhaps through shock-melting)
3. migration of free metal from olivine fayalitePure* iron end-member (Fe2SiO4) of the olivine solid solution series and an important mineral in meteorites. When iron (Fe) is completely substituted by magnesium, it yields the the pure Mg-olivine end-member, forsterite (Mg2SiO4). The various Fe and Mg substitutions between these two end-members are described based on their forsteritic (Fo) Click on Term to Read More and chromiteBrownish-black oxide of chromium and iron (Cr-Fe oxide), Cr2FeO4, found in many meteorite groups. Click on Term to Read More as a result of reductionOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons,  while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More processes (i.e., by reaction with graphiteOpaque form of carbon (C) found in some iron and ordinary chondrites and in ureilite meteorites. Each C atom is bonded to three others in a plane composed of fused hexagonal rings, just like those in aromatic hydrocarbons. The two known forms of graphite, α (hexagonal) and β (rhombohedral), have Click on Term to Read More), resulting in Mg-rich olivine and chromite and low-Ni metal
4. Cr acting as a chalcophile elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of Click on Term to Read More during reduction leading to its incorporation into troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites. Click on Term to Read More
5. close to chondritic bulk composition

Those meteorites which have undergone more extensive thermal processing and have lost their original geochemical and isotopic features (e.g., members of the HED suite) are then characterized as achondrites.

It has been determined that the H-chondrite parent body recently suffered three distinct collisional events at ~7.0, 22, and 33 m.y. ago, which are distinguished by a lower than normal 3He/38Ar ratio in the metal of those fragments ejected during the earlier events. These ejection events produced only weak shock effects (S1–S2) and radiogenic gas loss, but injected abundant fragments into Earth-crossing resonances.

From spectrographic data, the S(IV)-type asteroid 6 Hebe is thought to be a candidate for the parent body of the H-type ordinary chondrites. Hebe is a 116-mile-diameter asteroid located next to both the 3:1 and ν₆ resonances, providing an efficient and rapid transfer mechanism into Earth-crossing orbitThe elliptical path of one body around another, typically the path of a small body around a much larger body. However, depending on the mass distribution of the objects, they may rotate around an empty spot in space • The Moon orbits around the Earth. • The Earth orbits around Click on Term to Read More and a significant source of meteorites to Earth. It has been estimated that 6 Hebe could contribute ~10% of the meteorite flux to Earth and that it may be the source of one of the major ordinary chondrite groups. Models show that by mixing a component of 40% FeNi-metal with 60% H5 chondrite, an exact match to the spectra of 6 Hebe is produced. The IIE irons could have been created through impact-melting on the metal-rich H chondrite parent body to produce melt sheets or pods near the surface.

However, hydrocode models show inconsistencies exist between expected and observed CRE ages based on the scenario of direct injection into resonances. The steady delivery of H chondrite material from 6 Hebe to Earth also remains unexplained. Current studies by Rubin and Bottke (2009) have led to the conclusion that family-forming events resulting in large meteoroidSmall rocky or metallic object in orbit around the Sun (or another star). reservoirs, which have homogenous compositions and locations near dynamical resonances such as the Jupiter 3:1 mean motion resonance, are the likely source of the most prevalent falls including H chondrites and HED achondrites (especially howardites). As a matter of fact, a number of asteroids having H-like mineralogies have been observed near the 3:1 and 5:2 resonances at 2.82 AUThe astronomical unit for length is described as the "mean" distance (average of aphelion and perihelion distances) between the Earth and the Sun. Though most references state the value for 1 AU to be approximately 150 million kilometers, the currently accepted precise value for the AU is 149,597,870.66 km. The Click on Term to Read More (Burbine et al., 2015 and references therein). See further details on the Abbott page.

Northwest Africa 2898 shows evidence of having been very weakly shocked (stage S2, peak pressure of 5–10 GPa), and having experienced a low degree of weathering since its fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More (grade W1/2). The specimen of NWA 2898 shown above is a 1.6 g partial slice exhibiting a coarse-grained, recrystallized texture with many grains exhibiting 120° triple junctions. The photo below shows a magnified view of this H7 depicting the completely recrystallized texture.
Photo courtesy of Stefan Ralew—SR–Meteorite